Polypeptides and LHRH analogs in particular are historically administered parenterally because they are poorly absorbed by biological membranes due to their large molecular size and polarity, enzymatic degradation and deactivation by proteases enroute to the circulatory system. To improve bioavailability, some have developed formulations for rectal and nasal administration. These two routes of administration yield bioavailability results of about 0-5% and are not reproducible. Thus, these routes are pharmaceutically unacceptable.
Further, to date no aerosol formulation has been developed for administration of LHRH analogs by inhalation. This is due in part because many peptide drugs such as LHRH agonist and antagonist compounds do not appreciably dissolve in hydrophobic liquid vehicles to enable preparation of solution aerosols. Further, since suspension aerosols require micronization of the LHRH analogs, usually in air for efficiency reasons, and the LHRH analogs are biologically hazardous in low concentrations, suspension aerosols of LHRH analogs have not been considered feasible.
For example, leuprolide is a polar nonapeptide with three ionizable sites, namely the imidazolyl nitrogen of histidine with pKa approximately 6.0, the phenolic hydroxyl of tyrosine with pKa approximately 10.0, and the guanidine nitrogen of arginine with pKa approximately 13.0. Since the guanidine nitrogen is extremely basic, this nonapeptide as synthesized exists in the protonated form and is generally associated with at least one mole of acetic acid. Leuprolide, therefore, exists as an acetate salt, which is highly hydrophilic.
LHRH analogs are practically insoluble in fluorocarbons. In mixtures of ethyl alcohol and fluorocarbons, the solubility of leuprolide approaches 3 mg/ml which is not satisfactory due to dose requirements. This solubility estimate is not significantly affected by the presence of nonionic surfactants because, in part, of solubility and dielectric limitations of such surfactants. In mixtures of fluorocarbons, ethyl alcohol and water, experimental results showed equilibrium solubility of leuprolide to approach 5 mg/ml which is still unacceptable. At high concentrations of ethyl alcohol, a gel-like mass forms resulting in a colloidal dispersion that does not clear at room temperature for up to one month. At water concentrations of 10% or greater, a complete phase separation occurs making a homogeneous formulation impractical and renders aerosolization impractical.
Preparing suspension aerosols requires micronization of the drug prior to manufacture of the aerosol. This process involves mechanical breakup of the powder using grinding or milling equipment to reduce drug particle size to below 10 microns which is essential for pulmonary deposition of the aerosol. Generally, this milling process results in significant exposure of the drug to the surrounding environment as well as up to 20% loss of the drug. The airborne LHRH analog particles can cause safety and health hazards if precautionary measures are not taken.